Add Flatpak

[Zeckmathederg]

Flatpak is useful and helps eliminate the need to install a bunch of dependencies. A couple of issues:

• Flatpak itself requires a sizeable amount of dependencies, a good number already in the Shared Dependencies chapter
• Should have its own dedicated chapter to cover the dependencies that don't belong in Shared Dependencies
• Point to Flatpak as being an option to install Steam and avoid 32-bit lib requirements
• Will require some work to properly figure out and take a lot of time
• Adding more dependencies requires more work on the part of the GLFS team of editors in the future after Flatpak will be added

Benefits of adding Flatpak:

• It will be a good resource to use, to install Flatpak in an easy way on an LFS system instead of looking at Arch PKGBUILDs
• Flatpak is useful itself and many Linux applications are Flatpak only; the AppImage standard is fading and it's time to adapt
• BLFS does not have Flatpak so this will be a way of staying ahead of the game
• Does not require 32-bit libs

To me, it isn't much of a discussion, but rather a matter of actually doing it.